Basic Research Needs for Geosciences - Energetics Meetings and ...

PRIORITY RESEARCH DIRECTION:FLUID-INDUCED ROCK DEFORMATIONSCIENTIFIC CHALLENGESCommercial-scale carbon capture and geologic storage requires injection of large volumes ofimpure CO 2 into the shallow crust for many decades, and efficient trapping of the CO 2 forhundreds of years. The introduced buoyant fluid phase will partially displace ambient fluids,induce large local pressure transients, and establish new regional-scale multiphase fluid flowregimes. To be successful, the reservoir storage capacities must not be compromised by leakagethrough cap-rock faults and fractures that could be activated by the stress-strain response to theinjection-induced pressure evolution. Furthermore, maintenance of shallow groundwater qualitymust be ensured, and the hazards and risks associated with potential uplift and inducedseismicity must be quantified and managed. Hence, improved understanding of shallow crustaldeformation and multiphase fluid flow triggered by commercial-scale CO 2 injection is a priorityresearch need.The physics of coupled rock deformation and multiphase fluid flow has been a subject of studyfor many years, but still presents a substantial scientific challenge due to complexity of materialsand processes. Rock masses are highly heterogeneous with regard to lithology, texture, structureand strength, while the presence of multiscale discontinuities (faults, joints, and microfractures)has a significant impact on the coupled rheological and transport response to injection-inducedpressure and dependent stress perturbations. Under the ambient pressure and temperatureconditions most pertinent to CO 2 storage—generally, < 0.5 kb (< 5 km depth) and < 100 o C—thisoften nonlinear coupled response is particularly challenging to quantify because the rocksundergo brittle failure when stressed. Deeper in the Earth, where temperature and pressure arehigher, rock deformation is easier to model in some respects because the behavior is viscous orplastic, but still challenging to model when fluid is present and heterogeneously distributed, andmineral-fluid chemical reactions proceed at faster rates.Understanding geomechanical processes and properties at all scalesA multiplicity of deformation mechanisms at different scales may be activated by CO 2 injection.In porous clastic and carbonate rocks, grain-scale deformation such as microcracking, porecollapse, pressure solution, and crystalline plasticity may be operative. Although ourunderstanding of these processes in rocks saturated with a homogenous fluid is adequate (e.g.,Guéguen and Boutéca 2004), many questions remain for rocks saturated with multiphase fluids.For example, the mechanical effect of pore pressure changes on the elastic, inelastic, and failurebehavior of rocks saturated with a single fluid phase are commonly described using the effectivestress principle. To what extent this formulation applies to a rock saturated with brine and CO 2 isnot well known. Further, although it is likely that interfacial tension will influence the kinetics ofsubcritical crack growth and pressure solution in multiphase fluid-saturated rocks, thishypothesis requires demonstration. Finally, some studies have indicated that seismic velocity ishighly dependent not only on bulk CO 2 saturation, but also on how it is distributed in the porespace. Thus, systematic investigation of rock properties, including seismic velocity, seismicattenuation, and electrical conductivity in multiphase-fluid saturated rocks, will provide usefulinsights into the interpretation of geophysical imaging.The shallow crust is permeated with fractures on all scales. The mechanical stiffness, strength,and hydraulic transmissivity of these fractures depends on their aperture, roughness, and140 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems